Optical Parametric Oscillators
An Optical Parametric Oscillator (OPO) is a device employing one or more non-linear crystals which when pumped by a laser beam defining a pump wavelength, can generate coherent light at two different and longer wavelengths. The operation of an OPO typically requires a very high light intensity in the pump beam which is generally supplied by a very short pulse laser. In the OPO at least one non-linear crystal (such as BaB204, LiB305, LiNb03, KTiOP04 and others) is placed in an optical resonator. When the pump laser beam is directed to propagate through the crystal, a pair of beams (referred to as the signal beam and the idler beam) is produced. Energy of the photons in the beams is conserved so:
      1          λ      p        =            1              λ        s              +          1              λ        i            where λp refers to the wavelength of the pump beam, λs refers to the wavelength of the signal beam and λi refers to the wavelength of the idler beam. Typically the shorter wavelength beam is referred to as the signal beam and the longer wavelength beam is referred to as the idler beam. The generation of the parametric beams (the idler and the signal) in a single path through the crystal(s) is inefficient and only a small fraction of the pump beam is converted. In order to construct an efficient and useful device the crystal(s) are typically placed inside a resonator that is designed to oscillate one or both of the parametric beams inside the cavity, such that it (or they) are amplified in successive passes through the crystal(s). The oscillator components of the OPO are typically comprised of optical elements designed to provide the required feedback for efficient conversion. The principles of OPO are well known and described in many publications on lasers and non-linear optics (for example, A. Yariv, Quantum Electronics, 3rd edition, p. 411. John Wiley & Sons, New York). In many of these OPO's the wavelengths of the signal beam and therefore the idler beam can be tuned over a wide spectral range by varying the orientation of the crystal with respect to the laser beam, by changing the crystal's temperature, or by applying a variable voltage across the crystal. Various tuning ranges can be achieved by properly selecting the laser, the non-linear crystal, and the optical components.
Optical parametric oscillators (OPO's) have been recognized as critical devices for a wide range of applications. In the early stages they were used primarily for research applications and as the designs of these devices have improved they have been incorporated in instruments that are used in commercial applications, (e.g., medical Imaging and hyper-spectral Imaging). The transition from research to commercial applications present more stringent design constrains, and critical demands such as high reliability, high damage threshold, robust and compact designs. Although there have been significant advances in the design of OPO devices since they were first invented, a device that will meet all of these criteria is still needed.